Tensioning system for production tubing in a riser at a floating installation for hydrocarbon production

ABSTRACT

A device for tensioning and relieving production tubing ( 49 ) extending from a subsea hydrocarbon well ( 107 ) up through a riser ( 63 ) to a floating installation ( 87 ), a continuous production tubing extension ( 85 ) passing through a telescopic unit ( 67 ) in the upper part of the riser ( 63 ), preferably extending above a production deck ( 89 ) on the floating installation ( 87 ), where the production tubing ( 49 ) is provided with a tubing tensioner unit ( 1 ) below and in the proximity of the lower end of the telescopic unit ( 67 ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/NO2003/000384, filed Nov. 13, 2003, whichinternational application was published on Jul. 8, 2004 as InternationalPublication WO 2004/057147. The International Application claimspriority of Norwegian Patent Application 20025556, filed Nov. 20, 2002.

BACKGROUND OF THE INVENTION

This invention regards a tensioning device for production tubinginstalled in a riser connecting a subsea hydrocarbon well with afloating installation on the surface of the sea, in particular a systemfor tensioning the fixed part of the production tubing and relieving itby suspending it from the fixed part of the riser, and also maintainingtension in that part of the production tubing which passes through atelescopic riser unit up to the production deck, by connecting acounterweight to the production tubing by means of a wire suspended froma pulley in the derrick.

During test production from a subsea hydrocarbon well by use of afloating installation in the form of a drilling platform or a drillingship, the well and the floating installation are usually interconnectedby several connecting pipes, among other things production tubingthrough which hydrocarbons flow up from the well. Several pipes may begathered in a so-called riser of a dimension suitable for the purpose.

The actual riser is kept under tension by means of a riser tensionerattached to the floating installation. In order to compensate formovements of the floating installation caused by waves, tide, ballasttrimming etc., the upper section of the riser comprises a telescopicsection, which in some cases is combined with the riser tensioner. Someof the pipe connections inside the riser may have a similar telescopicsection.

Under normal circumstances, it is not desirable for the productiontubing to comprise a telescopic section. As a result, this is normallypassed rigidly through the telescopic section of the riser and up to theproduction deck of the installation, where it is suspended from thetensioning system via wires, which tensioning system consists of pulleysand cylinders interconnected in a manner such as to maintain the tensionin the production tubing regardless of the movements of the installationon the surface of the sea.

If the production tubing is not kept under tension, it may buckle underits own weight and possibly collapse, causing leakage. For this reason,the demands placed on the tensioners used on production tubing duringthis test phase are quite stringent. At the same time, the tensionersoften comprise large, complex components that are demanding in terms ofmaintenance and which require a lot of space in the central areas aroundthe production deck on the installation. For reasons of safety, it isoften made use of dual systems. Consequently, the production tubingtensioners also represent a great load on the installation, with arelatively high centre of gravity.

The object of the invention is to remedy the disadvantages of prior art.

SUMMARY OF THE INVENTION

The object is achieved by characteristics given in the description belowand in the following claims.

The tension in a riser is maintained in a manner that is known per se,by use of a tensioner comprising e.g. a tension collar attached to anupper section of the riser and further connected via wires to a systemof pulleys and cylinders suitable for maintaining a prescribed tensionin the riser. Alternatively, the tension may be maintained by a combinedtelescopic and tensioning device mounted in the extension of the upperpart of the riser.

A tubing tensioner unit is interposed between a telescopic section andthe upper section of the riser. A connection from the well via theproduction tubing passes centrally through the tubing tensioner unit.The tubing tensioner unit primarily comprises a housing and a tubularcylinder.

The housing of the tubing tensioner unit is essentially a concentrictube, the end portions of which are provided with flanges suitable forcomplementary connection to the upper section of the riser located belowand the lower end of the telescopic section located above. Said partsare rigidly mounted to each flange connection with the use of severalbolts.

The tubular cylinder comprises a piston section and a cylinder section.

The piston section comprises a tube with an inner diameter essentiallyequal to the dimension of the production tubing for which the tubingtensioner unit is intended. A middle section of the tube is providedwith a concentric piston with a diameter considerably greater than theouter diameter of the tube. The piston section is connected to theproduction tubing sections located above and below, by use of threadedconnecting sockets that are known per se. The piston casing is providedwith appropriate packings.

The cylinder section comprises a cylinder and lower and upper gableswith associated packings in concentric openings that match the outerdiameter of the piston section. The inner diameter of the cylinder isadapted to the outer piston diameter of said piston section. Preferably,the upper gable of the cylinder section is removable through beingprovided with male threads that correspond to female threads in thecylinder. The tube and its piston are arranged inside the cylindersection with the ends of the tube projecting from the gables of thecylinder. The piston divides the cylinder into an upper and a lowerchamber.

The lower gable of the cylinder section comprises a cylindrical portionthat extends down from the end of the cylinder. The casing of thecylindrical portion is provided with several recesses each designed toaccommodate a ratchet suspended in a swivelling manner from the upperend portion of the ratchet. The ratchet swivels out primarily in aradial direction from the cylindrical gable portion, so that the lowerportion of the ratchet projects from the gable portion. Typically, thelower end face of the ratchet points downwards at an angle and out fromits border against the inside face of the ratchet. The ratchets areessentially spaced evenly and concentrically around the gable portion.Each ratchet is provided with means of exerting a pressure against theinside of the ratchet, e.g. in the form of a compression spring, so thatthe ratchet swings out from its recess when the space outside theratchets is unobstructed.

The cylinder section is provided with an inlet for supply of fluid, e.g.hydraulic oil, to the lower chamber of the cylinder. The inlet isconnected to a hydraulic unit that is known per se, complete with pump,reservoir and control devices, in a manner that is known per se.

The inside wall of the housing of the tubing tensioner unit is providedwith an annular recess designed to leave room for the protrudingratchets of the gable portion when the cylinder has been inserted intothe housing of the tubing tensioner unit. A lower edge of the recessfaces down and out at an angle from its border against the housing wallsurface. The orientation of the lower edge typically coincides with thelower end face of the ratchets when the ratchets have been swung intothe annular recess.

When the tensioning system according to the invention is used, thehousing of the tubing tensioner unit is coupled to the riser at asuitable height above the seabed, whereupon the telescopic section iscoupled to the upper end of the housing by means of the appropriateflanges. When the well is to be tested, the production tubing isassembled and lowered successively through the riser. The tubularcylinder is connected to the production tubing at an appropriatedistance from the lower end of the production tubing, to allow thetubular cylinder to assume its position in said housing when the lowerend of the production tubing is connected to the wellhead installationon the seabed. The axial position of the cylinder on the productiontubing, which has been extended by the piston section of the tubularcylinder, is adjusted so as to make the ratchets on the tubular cylinderengage the annular recess in the housing.

When the ratchets of the tubular cylinder are engaged with the housing,the lower chamber of the tubular cylinder is pressurised hydraulicallyby means of the hydraulic unit. This tensions the production tubing, asthe reaction forces from the tubular cylinder are transferred to theriser via the ratchets and the housing of the tubing tensioner, which isplaced at the top of the riser.

Connected to the top of the riser is a production tubing extension whichruns continuously through a telescopic unit that is known per se, and upabove the production deck of the installation.

The upper end portion of the production tubing extension is connected toone end of a wire in a manner that is known per se. The wire is passedover a pulley device arranged considerably higher than the productiondeck, e.g. in the derrick. The other end of the wire is connected to afreely suspended counterweight with a mass adapted to the weight of theproduction tubing extension.

With this, a constant tension is maintained in the production tubingextension independently of the movements of the installation caused bywaves, tide, ballast trimming etc. The tension is maintained by means ofa simple and light-weight mechanism.

BRIEF DESCRIPTION OF THE DRAWING

The following describes a non-limiting example of a preferred embodimentillustrated in the accompanying drawings, in which:

FIG. 1 shows a longitudinal section through a tubular cylinder connectedto production tubing and about to be lowered through a riser, which iscoupled to the housing of a tubing tensioner unit;

FIG. 2 shows a longitudinal section through a tubing tensioner unitconnected to the riser, on the same scale;

FIG. 3 shows a longitudinal section through the tubular cylinder, on thesame scale;

FIG. 4 shows a longitudinal section through the housing of the tubingtensioner unit, on a larger scale; and

FIG. 5 shows a schematic drawing of a tensioning system according to theinvention on a smaller scale, here shown in cross section through adrilling ship.

In the drawings, reference number 1 denotes a tubing tensioner unitconsisting of a tubular cylinder 3 and a housing 5, see FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following, reference is made to FIG. 3. The tubular cylinder 3consists of a cylinder 7, an upper gable 9 and a lower gable 11. Bothgables 9, 11 have a central circular opening 13 provided with severalpackings 15. The upper gable 9 is provided with a through air passage 17and a hydraulic passage 19 for introduction of hydraulic fluid to thetubular cylinder 1. The hydraulic passage 19 follows the wall of thecylinder 7 and ends up in the tubular cylinder by the lower gable 11.

The lower gable 11 extends cylindrically out from the cylinder 7 in theaxial direction, with a diameter that substantially is slightly largerthan the outer diameter of the cylinder 7. The jacket 21 of the lowergable 11 is provided with several uniform cutouts 23.

In each cutout 23 a ratchet 25 is suspended from the gable 11 in aswivelling manner with an axle 27 through an upper part 29 of theratchet 25. A longitudinal section through the ratchet and in the radialdirection of the gable 11 shows the thickness of the ratchet 25 asdecreasing from the lower part 31 to the upper part 29. An inner ratchetface 33 is provided with a recess. 35. A lower ratchet face 37 extendsaway from the inner ratchet face 33 and partly downwards at an angle.The cutout 23 has a recess 39 corresponding to the recess 35 in theratchet 25. A spring device 41 capable of forcing the ratchet out isarranged with its ends in the recesses 35, 39.

A tubular piston rod 43 with end portions 45 threaded at both ends hasan internal diameter of the same order as on a production tubing 49. Theend portion 47 of the production tubing 49 is threaded correspondingly,and two corresponding end portions 45, 47 are joined with a sleeve 48provided with female threads. In a middle section 51 the piston rod isprovided with a concentric piston 53 that corresponds to the innerdiameter of the cylinder 7. The piston 53 is provided with appropriatepackings 55.

Reference is now made primarily to FIGS. 2 and 4. The housing 5 of thetubing tensioner unit comprises an essentially tubular main section 57plus a flange 59, 59′ placed at either end, which corresponds with aflange 61 on a riser 63 and a flange 65 on a telescopic unit 67. Theflanges 59, 59′, 61, 65 are connected with several through bolts 66.Internally, the main section 57 of the housing 5 is provided with anannular concentric recess 69 positioned approximately halfway betweenthe flanges 59, 59′ of the housing 5. A lower face 71 of the recessslopes slightly downwards from the inner wall 73 of the housing, so thatthe direction of the face 71 coincides with that of the lower ratchetface 37 (see FIG. 3) when the ratchet 25 is forced out from the cutout23 in the lower gable 11.

Reference is further made to FIG. 3. The tubular cylinder 1 is connectedvia the hydraulic passage 19 to a hydraulic unit 75 that is known per se(shown schematically), and which comprises pipe 77, reservoir 79, pump81 and control device 83.

Reference is now made to FIGS. 2 and 5. A production tubing extension 85is connected to the upper end portion 45 of the tubular cylinder 3 andprojects above the production deck 89 of a floating installation 87,where the tubing extension 85 is coupled to a flexible hose 91 and apulley 93. A wire 95 is attached to a tower 99, e.g. a derrick, by itsfirst end 97, and is then passed down to and through the pulley 93 andup to and over a fixed pulley 101, and then down in the direction of theproduction deck 89. The other end 103 of the wire 95 is attached to acounterweight 105.

A riser 63 extends from a wellhead installation 107 on the seabed 109and up towards the installation 87 floating on the surface of the sea111. The riser is suspended from several tensioning wires 115 at acollar 113, which wires are connected via pulleys 117 to risertensioners 119. The tension in the wires 115 is maintained by means ofregulating devices (not shown).

The housing 5 is connected by its one flange 59 to the flange 61 on theriser 63, through use of bolts 66. The riser 63 is then completed withthe telescopic unit 67 that is connected to the upper flange 59′ of thehousing 5 by means of bolts 66.

The riser 63 is kept under tension by means of the riser tensioners 119.A prescribed tension in the wires 115 is maintained by means of theregulating devices (not shown) that actuate the riser tensioners 119.

The production tubing 49 passes from the floating installation 87 downthrough the riser 63 to the wellhead installation 107. The tubularcylinder 3 is connected to the upper end of the production tubing 49 bymeans of the corresponding coupling devices 45, 47. Before connectingup, the length of the production tubing 49 is adjusted in relation tothe position of the housing 5 on the riser 63, so that the ratchets 25on the tubular cylinder 3 to all intents and purposes correspond withthe annular recess 69 of the housing 5 when the production tubing 49 islowered all the way to the wellhead installation 107 on the seabed 109.The production tubing extension 85 is connected to the upper end of thetubular cylinder 3 by means of the corresponding coupling devices 45′,47′.

The cylinder 7 with its flanges 9, 11 is then displaced up or down alongthe piston rod 43 until the ratchets 25 are forced out into the internalrecess 69 of the housing 5 and abut the lower faces 71 of the recess 69.

The upper end of the tubing extension 85 is connected to thecounterweight 105, so as to allow the counterweight 105 to keep thetension in the production tubing extension 85 by means of the pulleys93, 101 and the wire 95.

The tubular cylinder 3, which is connected up to the hydraulic unit 75,is then pressurised. The piston 53 will subsequently seek to stretch theproduction tubing 49, the ratchets 25 ensuring that the cylinder 7 andits gables 9, 11 rest on the face 71 in the housing 5. The reactionforces from the tubular cylinder 3 are thereby transferred to the riser63. The part of the production tubing 49 extending from the hydrocarbonwell 107 up to the tubing tensioner unit 1 is stretched and suspendedfrom the riser 63. Consequently only the upper part of the productiontubing 49, i.e. the production tubing extension 85, has to be suspendedfrom a tensioning device on the floating installation 87, and then inthe form of the simple arrangement consisting of the counterweight 105,the wire 95 and the pulleys 93, 101.

1. A device for tensioning and relieving production tubing, theproduction tubing extending from a subsea well up through a riser to afloating installation, wherein an extension of production tubing passesthrough a telescopic unit in an upper section of the riser, the devicecomprising: a tubing tensioner disposed below and in the proximity ofthe lower end of the telescopic unit, the tubing tensioner comprising acylinder section and a housing that is integrated in the riser, whereinthe cylinder section comprises a piston and cylinder that aretelescopically received by the housing; the cylinder having plurality ofratchets adapted to engage an annular internal recess formed in a lowerinner face of the housing; the piston having an open inner diameter thatforms a conduit between the production tubing and the production tubingextension and an outer diameter that is greater than the outer diameterof the production tubing; and an inlet for supply of pressurized fluidto a lower chamber of the cylinder.
 2. The arrangement of claim 1,wherein the housing is a concentric tube having end portions providedwith flanges suitable for complementary connection to the upper sectionof the riser and a lower end of the telescopic section.
 3. Thearrangement of claim 1, wherein the cylinder comprises lower and uppergables.
 4. The arrangement of claim 3, wherein the lower gable comprisesa plurality of recesses that each accommodate one of the plurality ofratchets, an upper end of each ratchet suspended in a swiveling mannerfrom the gable such that the lower end of each ratchet swivels outprimarily in a radial direction from the gable to project from thegable.
 5. The arrangement of claim 3, wherein each ratchet comprisesmeans for biasing the ratchet outward relative to the gable.
 6. Thedevice of claim 1, wherein an upper end portion of the production tubingextension is coupled to an above-sea tensioning apparatus.
 7. The deviceof claim 6, wherein the above-sea tensioning apparatus comprises severalpulleys, one or more wires, and one or more counterweights.